Differential conductivity-measuring apparatus

ABSTRACT

Apparatus for measuring the change and rate of change in electrical conductivity in a test system due to a chemical reaction between a substance to be detected and a test reagent, as, for example, in an enzyme-substrate reaction, said apparatus including a pair of probes each of which has a pair of electrodes, the electrodes of one of said probes having the test reagent associated therewith by matrix means, preferably in fixed form, such that when said probes are placed in contact with an ionic medium containing the substance to be detected, the conductivity between the electrodes is dependent upon the conductivity of the matrix means and/or said ionic medium, and conductivity-measuring circuit means connected to both probes which electronically subtracts the background conductivity caused by the ionic medium and provides a differential measurement of the change and rate of change in conductivity caused by the reaction of the substance to be detected with the test reagent and thereby provides a measurement of the concentration of the substance to be detected in the ionic medium.

United States Patent Rogers [15] amet 51 Jan. 11%, W72

[54] DIFFERENTIAL CONDUCTIVITY- MEASURING APPARATUS [72] Inventor:Robert Wayne Rogers, Elkhart, Ind.

[73] Assignee: Miles Laboratories, Inc., Elkhart, Ind.

[22] Filed: Nov. 13, 1969 [21] Appl. No.: 876,350

[52] U.S. Cl. ..23/253 R, 195/1035, 204/195 [51] Int.Cl..B01k3/00,C12k1/10,G0ln31/14 [58] Field oISearch ..23/253;l95/lO3.5;204/195 R,

FOREIGN PATENTS OR APPLICATIONS 1,054,626 l/1967 Great Britain ..324/3OB OTHER PUBLICATIONS Clark et 211., Annals New York Academy of Sciences,Vol. 102, Article 1, Pages 29- 45, Oct. 31, 1962, Page 40 relied on.

Primary Examiner-Morris O. Wolk Assistant Examiner-41. M. ReeseAttorney-Joseph C. Schwalbach, Michael A. Kondzella, Louis E. Davidsonand Harry T. Stephenson [57] ABSTRACT Apparatus for measuring the changeand rate of change in electrical conductivity in a test system due to achemical reaction between a substance to be detected and a test reagent,as, for example, in an enzyme-substrate reaction, said apparatusincluding a pair of probes each of which has a pair of electrodes, theelectrodes of one of said probes having the test reagent associatedtherewith by matrix means, preferably in fixed form, such that when saidprobes are placed in contact with an ionic medium containing thesubstance to be detected, the conductivity between the electrodes isdependent upon the conductivity of the matrix means and/or said ionicmedium, and conductivity-measuring circuit means connected to bothprobes which electronically subtracts the background conductivity causedby the ionic medium and provides a dif ferential measurement of thechange and rate of change in conductivity caused by the reaction of thesubstance to be detected with the test reagent and thereby provides ameasurement of the concentration of the substance to be detected in theionic medium.

SUPPLY 7 DIFFERENTIATOR E E PATENTEB JAN! 8 I972 SHEET 1 OF 3 PATENTEDJAN: 8 I972 SHEET 3 [1F 3 lll DIFFERENTIAL CONDUCTIVITY-MEASURINGAPPARATUS BACKGROUND OF THE INVENTION The present invention relates todifferential conductivitymeasun'ng apparatus, and more particularly, butnot limited to apparatus for continuously measuring the change and rateof change of conductivity in a test system during a chemical reactionbetween a substance to be detected and a test reagent. The apparatus mayalso be used to measure the progress of a chemical reaction in achemical process system.

As used herein, the following terms and definitions apply: ionic mediumis any fluid, solution, suspension, emulsion, gel, solid or other systemor combination thereof containing ionic species and which is capable ofconducting an electrical current; the substance to be detected is achemical material contained in the ionic medium and which is beingestimated or monitored; the test reagent is a chemical compound orcombination of chemical compounds which when contacted with thesubstance to be detected causes a change in conductivity in the system;matrix means is a gel or other chemical or physical means used to fix,immobilize or contain the test reagent in the desired relationshipbetween the ionic medium and the electrodes used to measureconductivity; test system is the total combination of ionic medium, thesubstance to be detected, test reagent and matrix means; electrode isdefined as an individual highly electrically conductive means having adefinite physical shape and when spaced from another electrode enablesthe obtention of conductivity data therebetween; and, probe is definedas a physical assemblage of a pair of spaced electrodes and any matrixmeans associated therewith, which matrix means may also incorporate atest reagent.

Techniques for making quantitative chemical determinations andestimations through measurement of the change in conductivity of a testsystem before and after reaction of the substance to be detected with atest reagent are generally known. For example, one method or thequantitative determination of an enzyme or substrate which involves themeasurement of the change in electrical conductivity of the test systemresulting from interreaction of the enzyme and substrate is described inan article entitled Conductivity Method for Determination of Urea by W.T. Chin et al., published in Analytical Chemistry, Nov. l96l, at pp.l,757-l,760. This method measures the electrical conductivity in a testfluid containing urea before and after reaction with urease andthereafter requires calculation of the change in conductivity. Anothermethod for analytically studying either of the elements of anenzyme-substrate reaction wherein the test fluid has a differentconductivity upon the addition of a test reagent is disclosed in U.S.Pat. No. 3,421,982. The latter method and apparatus exhibit certaindisadvantages which the subject invention has substantially overcome. Inparticular, the apparatus disclosed in US. Pat. No. 3,421,982 includescircuitry which requires continuous manual adjustment to obtain ameasurement of the change in conductivity; moreover, the method is basedon the assumption that the reaction produces a linear rate of change ofconductivity.

The present invention provides a differential conductivitymeasuringapparatus which is a substantial improvement over the known apparatusand techniques for measuring the change and rate of change inconductivity in a test system wherein a substance to be detected and atest reagent interreact.

SUMMARY OF THE INVENTION One of the primary objects of the presentinvention is to provide a novel method and apparatus for measuring thechange and rate of change in electrical conductivity in a test systemduring a chemical reaction between a substance to be detected and a testreagent.

Another object of the present invention is to provide an apparatus ofthe class described which is adapted to subtract the backgroundconductivity of an ionic medium containing the substance to be detectedfrom the measured change in conductivity caused by a chemical reactionbetween a test reagent and a substance to be detected.

Another object of the presentinvention is to provide a method andapparatus as described which automatically and continuously balances outor subtracts the background conductivity of an ionic medium from themeasured change in conductivity in a test system during a chemicalreaction between a test reagent and a substance to be detected.

Another object of the present invention is to provide a measuringapparatus as described, which includes means for continuously measuringthe rate of change in conductivity in a test system during a chemicalreaction between a test reagent and a substance to be detected.

In carrying out the above objects of the present invention, a measuringinstrument is usually employed which comprises a pair of probes, each ofwhich includes a pair of spaced electrodes. The electrodes of the firstprobe are in intimate contact with a test reagent, such as an enzyme,incorporated with a matrix means. The electrodes of the second probe arepreferably also in intimate contact with a matrix means, although suchmeans need not necessarily be employed; how ever, such electrodes arenot incorporated with the test reagent. In use, the first and secondprobes are contacted with an ionic medium containing the substance to bedetected. The conductivity measured by the second probe is dependentupon the conductivity of the matrix means associated with the electrodes of said probe and of the ionic medium and the conductivity of thefirst probe is dependent upon the conductivity of the ionic medium andof the matrix means associated with electrodes of the first probe, aswell as upon any change in conductivity produced by a chemical reactionbetween the test reagent and the substance to be detected. The apparatusincludes circuit means comprising positive and negative DC input stages,an oscillator circuit, a summing amplifier circuit, a differentiatorcircuit, and meter circuits, said circuit means being coupled to theprobes and adapted to subtract out the background conductivity of theionic medium contacted by the probes and to measure the change and rateof change in conductivity of the test system.

Further objects and advantages of the invention, together with theorganization and manner of operation thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several figures of which like referencenumerals identify like elements.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of themeasuring apparatus in ac cordance with a preferred embodiment of thepresent inventron;

FIG. 2 is a circuit diagram of the power supply for the measuringapparatus of FIG. ll;

FIG. 3 is a circuit diagram showing the audiofrequency oscillatorcircuit for the measuring apparatus illustrated in the block diagram ofFIG. 1;

FIG. 4 is a circuit diagram showing the measuring circuit used inconjunction with the power supply and oscillator circuits of FIGS. 2 and3, respectively; and

FIG. 5 is a perspective view of one form of probe which may be used withthe circuitry of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings, andin particular to FIG. ll, a differential conductivity-measuringapparatus in accordance with a preferred embodiment of the invention isshown in block diagram form and indicated generally by reference numeral It). The apparatus Ill includes probes Ill and 112 each comprisinga pair of electrodes each of which is connected through a functionswitch, indicated at 141, to an associated input stage circuit I6 or M,respectively. The function switch Ml selects the mode of operation ofapparatus It as will be more fully described hereinbelow. An oscillatorcircuit is connected to the input stage circuits 16 and 18, the outputsof the input stage circuits being connected to a summing amplifiercircuit. The output of the summing amplifier circuit 22 is connected toa meter circuit 24 for reading the change in conductivity between theelectrodes of a selected probe. The output from the summing amplifiercircuit is also connected to a differentiator circuit 26, the output ofwhich is connected to a meter circuit 28 for indicating the rate ofchange of conductivity between the electrodes of a selected probe. A DCpower supply, indicated generally at 30, uses 1 10 v. line voltage andprovides :6 v. signals to the circuit in a manner described more fullybelow.

With reference to FIG. 2, the power supply 30 has terminals 32 forconnection of a conventional source of alternating current, such as a115 v., 60 cycle, across the primary 34 of a transformer 36. Thesecondary 38 of the transformer 36 is centertapped at 40, the tap beingconnected through a conductor 42 to a terminal 44. One side of thesecondary 38 of transformer 36 is connected through a diode 46 to aresistor 52. The other side of the secondary 38 of transformer 36 isconnected through a diode 48 to the junction of diode 46 and resistor52. A capacitor 50 is connected between the junction of diode 46 andresistor 52 to the terminal 44 of the power supply through conductor 42.The resistor 52 is connected through a resistor 54 to a terminal 56. Azener diode 58 is connected between the junction of resistors 52, 54 andthe conductor 42. A pair of zener diodes 62 and 64 are connected inseries across the output terminals 56 and 44, and the junction of diodes62 and 64 is connected to a terminal 60. The power supply 30 provides 12v. DC between terminals 56 and 44, while providing :6 v. DC between theterminal and either of terminals 44 and 56.

Referring to FIG. 3, the oscillator circuit 20 comprises a field-effecttransistor-regulated phase-shift oscillator and buffer amplifier whichsupplies a sinusoidal voltage regulated to 0.1 v. peak-to-peakamplitude. The oscillator circuit 20 includes a resistor 66 connected tothe gate 68 of a field-effect transistor 70 and to the terminal 60 ofthe power source 30 through a conductor 72. A capacitor 74 is connectedfrom the gate 68 of transistor 70 to the conductor 72. The source 76 oftransistor 70 is also connected to the conductor 72. A diode 78 has itscathode 80 connected to the gate 68 of the field-effect transistor 70and has its anode connected to the drain 82 of a transistor 84 through aconductor 86. The anode of diode 78 is also connected to the output lead88 of an amplifier 90 through a conductor 92. The source 94 and gate 96of transistor 84 are connected through a resistor 98 and a variableresistor 100 to the negative input 102 of the amplifier 90 and the drainof transistor 70, which, in turn, are connected to conductor 72 throughresistor 104.

A resistor 106 and a capacitor 108 are connected in parallel to form aparallel resonant circuit. The parallel resonant circuit is connectedfrom the conductor 72 to the positive input 110 of the amplifier 90. Aresistor 112 and a capacitor 114 are connected in series between thepositive input 110 of amplifier 90 and the output lead 88 thereof andform a series resonant circuit. The resistors 106 and 112, andcapacitors 108 and 114 determine the frequency, which is 3 kHz. in thepreferred embodiment of the oscillator circuit. A resistor 116 isconnected between the output 88 of amplifier 90 and the positive inputof an amplifier 120, the positive input being connected to the conductor72 through a resistor 122. A conductor 124 connects the negative inputof the amplifier to the output 126 therefor and to the case ground 128of the measuring instrument.

Referring now to FIG. 4, the sensor means comprises first and secondprobes 11 and 12 comprising pairs of electrodes 134 and 138 and 144 and148, respectively. The probe 11 will be referred to hereinafter as theinput measuring probe and the probe 12 will be referred to as thereference or background conductivity measuring probe. The probes 11 and12 are constructed in such a way that the electrodes thereof extendbeyond the end of the body thereof in spaced relation and can be causedto intimately contact a conducting ionic medium. Thus, impression of anelectrical voltage across the pairs of electrodes will cause currentflow between the electrodes dependent upon the conductivity of the ionicmedium and any matrix means providing a conducting path between thespaced electrodes. While the probes 11 and 12 may be of generally knownconstruction, a probe construction found particularly useful in carryingout the present invention is disclosed in copending application Ser. No.30,329, filed Apr. 20, 1970 which is a continuation-in-part ofapplication Ser. No. 835,658, filed June 23, 1969 and now abandoned. Theconstruction and use of such probes is described more fully hereinafter.

The electrode 134 of input measuring probe 12 is connected to theinstrument case ground 128 through a conductor 136. The other electrode138 of the probe 12 is connected through a conductor 142 to a fixedcontact a of a five-position switch 140 in the function switch 14. Insimilar fashion, electrode 144 of the reference probe 12 is connected tothe case ground 128 through a conductor 146, the other electrode 148 ofthe reference probe 12 being connected through a con ductor 152 to afixed contact a of a five-position switch 150 in the function switch 14.

In addition to the five-position switches 140 and 150, the functionswitch 14 includes five-position switches 154 and 156. The movableconductor arms of switches 140, 150, 154 and 156 are mechanically linkedfor simultaneous movement in a conventional manner such as in a switchcommercially available under the trade name Centralab PA 1013. Each ofthe movable conductor arms of switches 140, 150, 154 and 156 has fivefixed contacts associated therewith; the switch arm 140 having contacts140 a to 2 associated therewith, the contact 150 having contacts 150 ato e associated therewith, the switch arm 154 having contacts 154 a to eassociated therewith, and the switch arm 156 having contacts 150 a to eassociated therewith. The five positions of the switches 140, 150, 154and 156 correspond to operate, balance, oscillator adjust up scale,oscillator adjust down scale and rate calibration, when considered asthe switch arms are moved through positions engaging the associatedcontacts a to 2, respectively.

The function switch 14 also includes an internal calibration resistor158 connected from the case ground 128 to contacts 14% and 140s of theswitch 140. A second internal calibration resistor 160 is connectedbetween the case ground 128 and contacts 15% and 150d of the switch 150.The switch contacts 154 a to d of switch 154 are connected through aconductor 162 to the contacts 156 a to e of switch 156. A conductor 164is adapted to connect the contact 1542 of switch 154 to either of thepower source terminals 44, 56 or 60 through a rate calibration controlvariable resistor 166 and a resistor 168 connected in series therewith.A conductor 170 connects the contacts 156 a to e of switch 156 to theterminal 60 of the power supply 30. A rate calibration capacitor 172 anda normally closed rate calibration switch 174 are connected in parallelbetween the conductors 164 and 170 as shown. Switch 174 is opened, asshown, for rate calibration.

The input stage circuit 16 includes an operational amplifier 176, suchas a solid-state amplifier, operating as an inverting amplifier andhaving rectifiers in the feedback circuit to convert the output of theamplifier to a positive DC voltage proportional to the input current.The amplifier 176 has a negative input conductor 178 connected to themovable arm of switch 140 of function switch 14 through a conductor 180.The positive input 182 of amplifier 176 is connected to the arm ofswitch 154 of the function switch 14. The negative input conductor 178of the amplifier 176 is connected to the movable conductor arms of threemechanically connected five-position switches 184, 186 and 188, each ofwhich comprises a movable conductor arm and five fixed terminalcontacts; the arm of switch 184 being associated with fixed contacts 184a to e, the arm of switch arm 186 being associated with fixed terminalcontacts 186 a to e., and the arm of switch 188 being associated withfixed contact terminals 188 a to e.

The terminal contacts 18 1a. 1810 and 1842 of switch 181 are connectedthrough a resistor 1911 to the movable contact of a potentiometer 192.The terminal contacts 18 1b and 1814 are connected through a resistor191 to the movable contact of a zero control potentiometer 196. Thecontrol potentiome ters 192 and 196 are connected in parallel acrossconductors 198 and 21111 which in turn are connected, respectively, tothe output terminals 56 and 44 of the power supply 111 through resistors2112 and 2111, respectively. The switch 181 and potentiometers 192 and196 provide means for selecting the desired sensitivity of thedifferential conductivity-measuring apparatus 111 and also provide DCbalance or zero controls for balancing the operational amplifier 176.

The fixed terminal contacts 1860, 186c, and 1862 are connected through aresistor 2116 to an output terminal 2118 of the input stage circuit 16.The terminal contacts 186b and 186d are connected to the output terminal2118 through a resistor 210. The terminal contacts 188a, 188a and 188sare connected through a resistor 212 to the cathode of a diode 2141, theanode of which is connected to the output 216 of amplifier 176. Theterminal contacts 1811b and 188d are connected through a resistor 218 tothe cathode of diode 214. A diode 2211 has its cathode connected to theoutput 216 of amplifier 176 and its anode connected to the outputterminal 2118.

The input stage circuit 18 includes an operational amplifier 222operating as an inverting amplifier and having rectifiers in thefeedback circuit to convert its output to a negative DC voltageproportional to the input current. The amplifier 222 has a negativeinput 224 connected to the movable contact arm of switch 151) offunction switch 1 1 through a conductor 226. A positive input 227 of theamplifier 222 is connected to the movable contact arm of switch 156 ofthe function switch 14. The input stage circuit 18 includes twofive-position switches 228 and 2311 which are mechanically coupled forsimultaneous movement with the above-described switches 1841, 186 and188 ofthe input stage circuit 16.

The five-position switch 228 includes a movable contact arm and fixedterminal contacts 228 a to e, and the five-position switch 2311 includesa movable contact arm and five terminal contacts 2311 a to e. Thecontacts 228a, 2280 and 228:: are connected through a resistor 232 to anoutput terminal 234 of the input stage circuit 18. The contacts 2281)and 228d are connected to the output terminal contact 234 through aresistor 236. The contacts 2311a, 2300 and 2311s are connected through aresistor 238 to the anode of a diode 2410 which has its cathodeconnected to the output 2412 of the amplifier 222. The contacts 2311band 230d are connected to the anode of diode 2 111 through a resistor214. A diode 2 -16 has its anode connected to the amplifier output 242and its cathode connected to the output terminal 234.

The summing amplifier circuit 22 serves to combine the outputs from theinput stage circuits 16 and 18 and, as will become more apparenthereinbelow, includes a switch 272 for selecting the desired sensitivityand means for balancing the outputs from the input stage circuits tocompensate for electrode differences. Switch 272 is mechanically coupledfor simultaneous movement with switches 18 1, 186, 188, 228 and 2311 toform therewith a range switch 331. The summing amplifier circuit 22includes an amplifier 2511 having a negative input 252 and a positiveinput 254. The negative input 252 of the amplifier 2511 is connected tothe output terminal 2118 of the input stage circuit 16 throughseries-connected resistors 256 and 258. The negative input 252 of theamplifier 2511 is also connected to output terminal 234 of the inputstage circuit 18 through a balance control variable resistor 2611connected in series with a resistor 262. A capacitor 264 is connectedbetween the terminal 611 of the power supply 311 and a terminal 266between the series-connected resistors 256 and 258. A similar capacitor268 is connected between the terminal 611 of the power supply 311 and aterminal 2711 between the series-connected variable resistor 2611 andresistor 262.

The balance control variable resistor 2611 provides a means forbalancing the output from the input stage circuits 16 and 111 tocompensate for electrode differences.

The summing amplifier circuit 22 also includes a plurality of feedbackresistors and a range switch 272 having a contact arm movable to contactany one of five fixed terminals 272 a to e for selecting one of the fivefeedback resistors. The movable contact arm of switch 272 is connectedto the negative input 252 of the amplifier 2511. The fixed terminal27201 is connected through a variable resistor 27 1 to the output 276 ofthe amplifier 2511. The fixed contact 272b is connected through avariable resistor 278, having a value less than the variable resistor2741, and a resistor 2811 to the output 276 of the amplifier 2511. Thecontact 2720 is connected through a variable resistor 282, having arange intermediate that of variable resistors 271 and 278, and aresistor 28:1 to the output 276 of the amplifier 2511. The switchcontacts 272d and 272a are connected through variable resistors 286 and288, respectively, having ranges different than the ranges of scaleresistors 27 1, 2'78 and 272, to a terminal 2911 which, in turn, isconnected through a resistor 292 to the output 276 of the amplifier2511. The posi tive input 251! of the amplifier 2511 is connectedthrough a resistor 291 to the terminal 611 of the power supply 311.

The output 276 of the amplifier 2511 is connected to a terminal 296 of ameter-reversing switch 298 in the meter circuit 241. The meter circuit21 is adapted to read the output from the summing amplifier circuit 22as differential conductivity. To this end, the meter circuit 24 includesa linear scale meter 31111 such as a microarnmeter or other suitabledevice connected in circuit with terminals 3112 and 3111 of thereversing switch 298. A scale calibration variable resistor 3116 isconnected between the terminal 3112 of the reversing switch 298 and thepositive terminal of meter 31111 and provides a sen sitivity control forcircuit calibration. Terminals 295 and 297 are connected to terminals3118 and 296 respectively of reversing switch 298. A terminal 3118 ofthe reversing switch 298 is connected to the terminal 611 of the powersupply circuit 311.

A differentiating circuit 26 connected to the summing amplifier circuit22 is adapted to differentiate the output from the summing amplifier2511. The differentiator circuit 26 includes an operational amplifier312 having a negative input 311 and a positive input 316. The positiveinput 316 is connected through a resistor 318 and conductor 317 to theoutput terminal 611 of the power supply circuit 311. The negative input314 of amplifier 312 is connected through a series-connected capacitor3211 and a resistor 322 to the output 276 of the amplifier 2511 in thesumming amplifier circuit 22. Capacitor 3211 and resistor 322 serve as alow pass filter to exclude unwanted frequencies from thedifferentiator-connected amplifier 312. A feedback circuit including aparallel-connected capacitor 321 and a resistor 326 are connectedbetween the output 328 and negative input 311 of the amplifier 312 toprovide the time constant for the integrator.

The output 328 of the differentiator circuit amplifier 312 is connectedthrough a scale calibration variable resistor 3311 to the positiveterminal of a meter 332, such as a microammeter or other suitabledevice, the negative terminal of which is connected to the outputterminal 611 of the power supply circuit 311 through the conductor 317.The variable resistor 3311 serves as a calibration control for the meter332 which provides a reading of the output of the differentiator circuit26 as a rate of change of conductivity during measuring.

FIG. 5 illustrates a form of probe construction which may be utilized inprobes 11 and 12 in FIGS. 1 and 1. A. flat circular disc 111 ofplexiglass one-half inch in diameter has two holes found in the centerportion thereof, and the terminal portions of two 22 gauge platinumwires 1341 and 138 project therethrough. The terminal portions of wires131 and 138 are bent to overlay the surface of disc 111 and areparallel, l centimeter in length and spaced 2 millimeters apart. Leadwires 411 1 and are attached to platinum wires 1341 and 138, and extendaxially through a centrally bored 11-inch by b inch polystyrene handle416 which is cemented at one end to the plexiglass disc 411 in coaxialrelation thereto. A tygon sleeve 417 is fitted telescopically over thehandle 416 and projects axially beyond the outer surface of the disc 411to provide a cavity defined by inside wall 419 of the tip of tygonsleeve 417 for containing and mechanically supporting a disclike layerof matrix means 418. Such matrix means 418 may comprise a polymericsemipermeable membrane. In the case of input measuring probe 11 thematrix means 418 preferably contains a test reagent, such as an enzyme;and in the case of reference probe 12 the matrix means 418 need notcontain a test reagent.

OPERATION In using the apparatus described hereinbefore, the electrodesof the probes 11 and 12 are contacted with an ionic medium, for examplethe fluid to be tested. The oscillator supplies a voltage to the probes11 and 12. The input stages 16 and 18 sense the current flow across theelectrodes of the probes 11 and 12 and supply DC voltage of oppositepolarities to the summing amplifier 22 such voltages being proportionalto the conductivity at the electrodes of the respective probes 11 and12. The summing amplifier 22 supplies an output current which isproportional to the difference in conductivity at the electrodes of theprobes 11 and 12, said current being measured by the meter circuit 24,and the differential conductivity is indicated thereby in micromhos.

The output current of the summing amplifier 22 is fed into thedifferentiator circuit 26 which senses the rate of any change in theoutput current of the summing amplifier circuit 22 and supplies acurrent to the meter circuit 28 which is proponional to and indicatesthe direction of any such change. The meter circuit 28 indicates thechange in conductivity in micromhos per second.

What is claimed is:

1. Apparatus for measuring the change in electrical conductivity in atest system due to a chemical reaction between a test reagent and asubstance to be detected contained in an ionic medium comprising, incombination, sensor means including first and second probes each havinga pair of electrodes contacting said ionic medium, the electrodes ofsaid first probe having said test reagent incorporated therewith bymatrix means, and circuit means connected to both of said probes andoperable to differentially measure the change in conductivity of saidprobes resulting from a chemical reaction between said test reagent andthe substance to be detected when said probes are in contact with anionic medium containing said substance.

2. Apparatus as defined in claim 1 wherein said circuit means includesmeans for subtracting the conductivity sensed by said second probe fromthe conductivity sensed by said first probe.

3. Apparatus as defined in claim 1 wherein said circuit meanscontinuously subtracts the conductivity sensed by said second probe fromthe conductivity sensed by said first probe during said chemicalreaction, thereby affording a measurement of the rate of change of saiddifferential conductivity during said reaction.

4. Apparatus as in claim 1 wherein said test reagent is incorporated insaid matrix means and said matrix means is placed in intimate contactwith the electrodes of the first probe.

5. Apparatus as in claim 4 wherein said matrix means comprises apolymeric semipermeable membrane.

6. Apparatus as in claim 1 wherein the electrodes of said second probeare also in intimate contact with a matrix means.

7. Apparatus as in claim 1 wherein said circuit means comprises anoscillator connected to an electrode of each probe for supplyingelectrical power thereto, a first input circuit connected to the otherelectrode of said first probe and having a positive DC voltage output, asecond input stage circuit connected to the other electrode of saidsecond probe and having a negative DC voltage output, a summingamplifier connected to and operable to combine the output of said inputstages, a

meter connected to said summing amplifier and affording an indication ofdifferential conductivity at said probes.

8. Apparatus as in claim 7 wherein said circuit means also includes adifferentiating circuit connected to said summing amplifier and operableto measure the rate of change of the differential conductivity at theprobes, and a meter connected to said differentiating circuit andaffording an indication of said rate of change of conductivity.

2. Apparatus as defined in claim 1 wherein said circuit means includesmeans for subtracting the conductivity sensed by said second probe fromthe conductivity sensed by said first probe.
 3. Apparatus as defined inclaim 1 wherein said circuit means continuously subtracts theconductivity sensed by said second probe from the conductivity sensed bysaid first probe during said chemical reaction, thereby affording ameasurement of the rate of change of said differential conductivityduring said reaction.
 4. Apparatus as in claim 1 wherein said testreagent is incorporated in said matrix means and said matrix means isplaced in intimate contact with the electrodes of the first probe. 5.Apparatus as in claim 4 wherein said matrix means comprises a polymericsemipermeable membrane.
 6. Apparatus as in claim 1 wherein theelectrodes of said second probe are also in intimate contact with amatrix means.
 7. Apparatus as in claim 1 wherein said circuit meanscomprises an oscillator connected to an electrode of each probe forsupplying electrical power thereto, a first input circuit connected tothe other electrode of said first probe and having a positive DC voltageoutput, a second input stage circuit connected to the other electrode ofsaid second probe and having a negative DC voltage output, a summingamplifier connected to and operable to combine the output of said inputstages, a meter connected to said summing amplifier and affording anindication of differential conductivity at said probes.
 8. Apparatus asin claim 7 wherein said circuit means also includes a differentiatingcircuit connected to said summing amplifier and operable to measure therate of change of the differential conductivity at the probes, and ameter connected to said differentiating circuit and affording anindication of said rate of change of conductivity.